My present invention relates to a common antenna for primary and secondary radar systems.
It is frequently necessary in a radar station to combine a number of antennas in the same operating location. However, this causes problems because the equipment in question has to be located in an area which is extremely restricted in the case of, for example, weapon systems. The combination of a primary radar antenna and a secondary radar antenna can be realized in two different ways. In one instance the antenna of the secondary radar is separate from that of the primary radar, the antennas installed in this way being essentially of the "beam" type. In the other instance the antenna of the secondary radar is integrated into the primary radar antenna, thus bringing about a true bifunctional antenna for the primary and secondary radars.
A bifunctional antenna for primary and secondary radars is generally constituted by a single reflector illuminated by a confronting source in such a way as to radiate energy into space for the purpose of detecting a target such as an aircraft, this being called the primary radar function, and also to transmit an interrogation signal to an aircraft equipped with a transporter which automatically transmits its answer, this being called the secondary radar function.
The radiated beam carrying the interrogation signal is effective in the direction where the aircraft has been detected. However, it has been found that the transponder of the interrogated aircraft or possibly that of a different aircraft could be triggered by secondary lobes of the interrogation diagram, whose level is liable to be relatively high compared with that of the major lobe. To obviate this disadvantage, the single antenna referred to can be provided with supplemental radiating elements affecting the reception of the interrogation signal by the remote transponder as well as the reception of the answer from the latter by the local receiver; these elements radiate in accordance with a quasi-omnidirectional control diagram whose level is such as to blank the secondary lobes of the interrogation diagram.
This arrangement makes it possible, by comparing the amplitude of the pulses received from the transponder and those received from the control system in the associated circuits, to determine the pulse received in reply to the interrogation by the major lobe.
The means for establishing the control diagram and affecting the transmission of an interrogation signal as well as the reception of a response signal from an interrogated target must be so designed that the gain of the associated control channel is greater than that of the interrogation and response channel in the angular zones containing the secondary lobes of the directional interrogation diagram, but much smaller in the direction of its major lobe.
In existing constructions the control means comprise radiating members, namely wave emitters, whose radiation pattern is of the omnidirectional type, positioned on the common reflector close to its boresight axis or on its upper part. They may also serve as the transmission source of the interrogation signal emitted for a limited time in a directive radiation pattern.
However, despite these precautions the radiation pattern of the control means does not completely fulfill its function, either because it is not totally omnidirectional or because certain high-level secondary lobes of the main directional pattern are not blanked and also because in some instances the major lobe may have such a low level as not to be absorbed by the omnidirectional diagram. Moreover, the control diagrams are disturbed by certain external structures, such as for example radomes under which the antennas are placed.
Finally, all these additional members, such as wave radiators, cause masking phenomena of the primary source due to the shadow created by these radiators on the surface of the reflector.